1. Field of the Invention
The invention relates to the process of fabricating integrated circuits. More specifically, the invention relates to a method and an apparatus for fracturing polygons on masks used in an optical lithography process for fabricating integrated circuits.
2. Related Art
Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are forced to deal with problems that arise as a consequence of the optical lithography process that is typically used to manufacture integrated circuits. This optical lithography process begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are generally formed of chrome, and light-transmissive clear regions, which are generally formed of quartz, is then positioned over this photoresist layer. (Note that the term “mask” as used in this specification is meant to include the term “reticle.”) Light is then shone on the mask from a visible light source, an ultraviolet light source, or more generally some type of electromagnetic radiation source together with suitably adapted masks and lithography equipment.
This image is reduced and focused through an optical system containing a number of lenses, filters, and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, opaque regions of the mask block the light leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, through chemical removal of either the exposed or non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching underlying regions of the wafer.
The masks used to expose the photoresist layer are typically processed by an optical proximity correction (OPC) process to alleviate problems cause by the diffraction of the exposure radiation at the feature edges, and over-etching of the photoresist at the ends of the features. The OPC process adds elements such as serifs and hammerheads to the original polygons. These added elements (and some of the original polygons) can cause problems during the mask writing process because the mask writing equipment can typically print only rectangles and trapezoids. Hence, after the OPC process, mask features are typically “fractured” so that each exposure element is a rectangle or a trapezoid.
For example, FIG. 1A illustrates a polygon 102 with serifs 103 that have been added by the OPC process. Since polygon 102 is not a rectangle or a trapezoid, a fracturing process is performed on polygon 102 to slice polygon 102 into rectangles and trapezoids. This fracturing process occurs because of limitations within the mask writing equipment.
FIG. 1B illustrates a cut that is made during the process of fracturing a polygon into rectangles and trapezoids. During the fracturing process, a cut 104 from a vertex 105 is considered in an attempt to eliminate the cavity 107 in polygon 102. However, this cut is rejected because it creates a “sliver” 106, which is too small to be printed easily.
In an attempt to fracture the polygon into rectangles without creating slivers, designers developed a fracturing process that determines the smaller width of the two resulting shapes on each side of the slice. The system considers at all possible slices from a vertex in this way and selects the slice that produces the greatest smaller width. For example, FIG. 1C illustrates a slice 108 that is made using this technique. Note that the slice 108 is made instead of the sliver 106 in FIG. 1B because the width of resulting shape 109 is larger than the width of sliver 106 made by cut 104 illustrated in FIG. 1. This slicing technique eliminates the slivers. However, slicing the polygon 102 lengthwise in this way fractures critical dimension 112 into multiple rectangles, which can create registration problems during the multiple exposures required to expose the mask blank. This gives rise to unwanted variations in the critical dimension 112. Additionally, since some of the rectangles are relatively long, multiple exposures (shots) may be required to expose these rectangles.
A sliver is a rectangle or a trapezoid whose minimum width is below a user-defined threshold. The polygon-fracturing algorithm targets “shaped beam” electron photolithography, where the mask is exposed by photographic equipment, which directs the beam through a keyhole aperture. If this aperture is too narrow, the exposure will be less successful than normal. Machine parameters dictate what this minimum aperture width; any shape which falls below this minimum aperture width is termed a “sliver.”
Hence, what is needed is a method and an apparatus for fracturing polygons used in an optical lithography process without the problems described above.